oops, i copied-and-pasted that wrong. please ignore that last post. the
correct one is this:

the point of this discussion is to try to see what would happen if we take
the Bible at face value, minus the Fall, and assume no death if no Fall. so,
in answer to George's comment, 2 kids per couple is linear growth if nobody
dies.

as for where we get wives for Cain and Seth, consider the features of the
Bible-at-face-value model:
a) Eve and Adam are the first and only humans to be created by God without
reproduction
b) God created this couple to be "very good" like the rest of creation,
which i extrapolate to mean "without genetic flaws" among other things.

one consequence of point b) is that incest would not produce deformities in
children. a possible reason for the prohibitions against incest in the Torah
is that incest produces deformities and retardation. but if that is not a
worry in a genetically perfect world, the prohibition against incest would
not be needed.

so, a natural result of the Bible-at-face-value model is that Cain and Seth
(remember that Abel was killed - aren't i a nitpicker?? :P) married their
sisters and that was perfectly acceptable.

that's why i modified Jim's original model to each couple having 4 kids,
with incest occuring at least in the early generations.

but Jim, you've got a point in saying that we were likely made to have way
more kids. i read somewhere that a woman's ovaries contain on the order of
10^5 eggs! yikes! the best suggestion i can think of to allow everyone to
live forever is the one i gave earlier: that God must have meant for us to
space travel. boy, with all that space in the universe and all the cool
things to study out there, how could God *not* have meant for us to space
travel? another point to consider is that Jesus said that in the kingdom of
God, people don't marry. so maybe God ushers in the "kingdom" at some point,
somehow, during the history of our model civilisation (no death, no Fall).

so, in short, i feel that the Bible-at-face-value model (no death prior to
the Fall) *can* work with the addition of a reasonable free parameter (eg,
space travel, or a time-boundary condition).